Ion vibrational spectroscopy of the microhydrated iodate anion: unveiling the onset of heterogeneous ion solvation

Abstract

Gas-phase ion vibrational spectroscopy is combined with first principles calculations to provide a bottom-up view of how the iodate anion is microhydrated from one to twenty water molecules. Infrared photodissociation spectra of D₂-tagged IO₃⁻(H₂O)_n (n = 1–8 and 11) are reported in the O–H stretching (2800–3800 cm⁻¹) and the H–O–H bending (1600–1800 cm⁻¹) region. Additionally, infrared multiple photon dissociation spectra of bare IO₃⁻(H₂O)_n (n = 10–12, 15, and 20) were recorded in the O–H stretching region. Structure assignments are made by comparing experimental spectra with simulated ones derived from MP2 frequency calculations and ab initio molecular dynamics simulations. Two solvation regions around the trigonal pyramidal iodate anion are identified based on their characteristic vibrational signatures. The first water molecules prefer to interact with the negatively polarized O-atoms of iodate by hydrogen bond donation. Notably, interwater hydrogen bonds are first detected at n = 3 as part of a water trimer ring. Starting at n = 4, a water tetramer ring motif is found and it persists in all larger systems. The emergence of a free-OH oscillator feature (∼3713 cm⁻¹) signals the onset of I-atom solvation at n = 11, manifested in the form of I^δ+⋯OH₂ interactions as a result of the I atom's electropositive nature in IO₃⁻. The interwater hydrogen bonds associated with I-atom solvation are considerably stronger compared to those associated with O-atom solvation, resulting in characteristically red-shifted (<3000 cm⁻¹) O–H stretching bands. Overall, this study sheds new light on polyoxoanion solvation, complementing insights from condensed-phase experiments.